During more than two decades of development of electron-beam-accessed stored-charge memory devices, successively more satisfactory solutions to four major initial problems of such devices have been devised. These problems were, first, limited life of the cathodes of the electron beam sources; second, rapid fading of data charge patterns in the absence of refresh; third, difficulty of precisely accessing a given addressed location on the storage target; and fourth, limited packing density of data on that target.
To date, oxide cathodes for electron beam sources have been largely superseded by much longer-lived varieties such as dispenser and lanthanum hexaboride types. The cathode life problem is thus reasonably well solved, and can be completely solved when cold cathodes, such as the field emission or negative-electron-affinity types, are perfected. Rapid fading of data charge patterns has been cured by the development of superior insulating materials such as amorphous silicon dioxide, deposited by pyrolysis or grown by oxidation of a silicon target substrate.
The other two initial problems are still believed to require better solutions; also, existing solutions to each of them seem to exacerbate the other. While it is certainly possible simply to stabilize the mechanical and electrical parameters of an electron-beam-accessed device in open-loop fashion sufficiently to be able to re-access a given data location with any required precision, the resulting bulk and expense of the device and its auxiliaries become impractically great when the requisite precision is high and data packing density is great, such as, for example, substantially more than 100 data locations per storage target width. For this reason, schemes for interleaving location information elements with the data storage elements in data storage targets have been devised. The location elements, when accessed by the electron beam, yield location information which can be used in closed-loop fashion to correct and guide the beam deflection, thus permitting precise and reliable data access in spite of drift in equipment parameters. Typical examples of such prior art schemes are shown in U.S. Pat. Nos. 3,624,633 and 3,789,372 to which reference is made and incorporated herein as if set out at length.
Such interleaving of location information elements with data storage elements, however, considerably reduces the net data packing density of the target, and also adds to its complexity and cost, for a given data capacity. Moreover, since location information elements typically are fabricated as discrete features they are not fully compatible with smooth-layer targets such as the metal-oxide-junction structure described by William C. Hughes et al. in their paper "A Semiconductor Nonvolatile Electron Beam Accessed Mass Memory" in Proceedings of the IEEE, Vol. 63, No. 8, August 1975, pages 1230-1240 to which reference is made and incorporated herein as if set out at length. Since such smooth-layer targets can provide data packing density superior to that of targets with discrete fabricated areas, it is clear that such closed-loop beam guidance schemes are not fully compatible with great data packing density.
To avoid the limitation noted immediately above, another stratagem has been devised -- relaxing the requirements upon electrical and mechanical precision for open-loop data access by dividing the beam deflection process into two steps, first coarsely directing and focusing a beam upon one of a matrix of smaller focus-deflection elements, and then performing fine-scale deflection and sharp focus in the selected element. This is the so-called "matrix" or "fly's eye" lens system, described in the aforementioned Hughes et al paper and in U.S. Pat. No. 3,491,236. While this stratagem can yield precise access, the requirements upon the accuracy of the large number of matrix lens components, and their mechanical rigidity with respect to each other and their associated target surfaces, result in an undesirably expensive structure.